Beverage-producing machines, for example coffee and espresso producing machines, are often provided with a device for the production of frothed milk or hot milk. A milk frothing device usually comprises a milk suction chamber wherein a steam delivery nozzle delivers a flow of pressurized steam generated by a steam generator in the beverage-producing machine. A milk suction duct is in fluid communication with a milk container and with the milk suction chamber. A flow of hot, pressurized steam flowing through the milk suction chamber generates, by Venturi effect, a negative pressure in the milk suction chamber so that milk is sucked through the milk suction duct, mixed with the steam and heated by the latent vaporization heat contained in the steam, which is condensed in the milk flow. An air inlet port is further provided, in fluid communication with the milk suction chamber, so that an adjustable amount of air can be sucked into the milk path, if desired. The milk can thus be emulsified with air to produce frothed milk. An adjustable pin is usually provided, for adjusting the cross section of the air inlet port and thus the amount of froth. If the air inlet port is entirely closed by the pin, no air is sucked into the milk path and hot, unfrothed milk is generated. A frothing device of this kind is disclosed for example in EP0243326.
Milk frothing devices of this kind are efficient and flexible in use but suffer from same disadvantages mainly caused by the narrow cross section of the air inlet port. This causes often the risk of clogging of the air inlet port, for example due to milk entering the air inlet port.
In order to overcome these problems WO-A-2011/158171 discloses a novel kind of frothing device, wherein a large air passage or air inlet port is arranged along the milk path. The air inlet port is actually formed by an open space between two sequentially arranged pipes through which the milk flows. A large amount of air is entrained by the milk flow and is mixed with milk to generate milk froth. A cyclone chamber is arranged downstream the air inlet port for removing the excess of air from the frothed milk. This known device solves the problem of clogging of the air inlet port. However, this known device does not provide the same flexibility of use of other frothing devices, which are provided with an air adjusting pin. The lack of an air adjusting pin makes the device unsuitable for the production of hot, unfrothed milk.